Modern high-efficiency combustion turbines have firing temperatures that exceed about 2000° F. (1093° C.), and firing temperatures continue to increase as demand for more efficient engines continues. Many components that form the combustor and “hot gas path” (HGP) turbine sections are directly exposed to aggressive hot combustion gases, for example, the combustor liner, the transition duct between the combustion and turbine sections, and the turbine stationary vanes and rotating blades and surrounding ring segments. In addition to thermal stresses, these and other components are also exposed to mechanical stresses and loads that further wear on the components.
Gas turbine engines may be operated using a number of different fuels. These fuels are combusted in the combustor section of the engine at temperatures at or in excess of 2000° F. (1093° C.), and the gases of combustion are used to rotate the turbine section of the engine, located aft of the combustor section of the engine. Power is generated by the rotating turbine section as energy is extracted from the hot gases of combustion. It is generally economically beneficial to operate the gas turbine engines using the most inexpensive fuel supply available. Two of the more abundant and inexpensive petroleum fuels are crude oil and heavy fuel oil. One of the reasons that they are economical fuels is that they are not heavily refined. Not being heavily refined, they may contain a number of impurities.
Heavy fuel oils typically contain several metallic elemental contaminants entrained as organic or inorganic complexes. These metallic elements, which may include one or more of sodium, potassium, vanadium, lead, and nickel, interact with oxygen and sulfur during combustion, including oxidation in the combustion plume, to form reaction products, including low melting point oxides. Sodium and potassium are conventionally removed prior to being injected into the combustion chambers by using an upstream fuel oil treatment system. Elements, such as vanadium and lead, however, are difficult to remove from the fuel by upstream accessories means.
Even the more refined liquid fuels used to power gas turbines are often residuals from distillation processes and typically contain significant levels of several contaminant elements. The oxides of these contaminants form low melting point compounds that flux the protective oxide scales and cause rapid corrosion during combustion.
The reaction products of these contaminants are problematic for at least two reasons. First, sodium vanadate, vanadium oxide, sodium sulfate, potassium sulfate, and lead oxide are extremely corrosive for the hot gas path alloys, including nickel-based and cobalt-based superalloys. Second, significant amounts of inhibitors may be needed to neutralize these corrosive oxides, such as, for example, inhibitors that form relatively inert vanadates from vanadium. But it is well known that in spite of the use of inhibitors, the components still undergo corrosion.
The molten oxides formed from the metal impurities react aggressively with native oxides formed in the nickel-based and cobalt-based alloys and induce rapid hot corrosion. Thermal barrier coatings on the nickel-based and cobalt-based alloys may be used to try to protect the parts and reduce corrosion, but some molten oxides, including vanadium oxide, are able to attack and react with some thermal barrier coatings to remove or degrade the thermal barrier coatings.